Grinding mill and method of operation



Nov. 7, 1967 GYM. MEISEL GRINDING MILL AND METHOD OF OPERATION Filed Aug. 23. 1965 IV on l ||l1 v mm film mm \m R 1 H J h Em mm on n 1- u u u .111 l L mm Till! lllll n n m i 1 w u F: FQN. Wm I v fi M T :1. lililwwlxXTlll ai i JQ llllll f INVENTOR.

GEORGE M. MEISEL BY 7 E ATTORNEY United States Patent 3,351 293 GRINDING MILL AND METHOD OF OPERATION George M. Meisel, El Cerrito, Califi, assignor to Kaiser Industries Corporation, Oakland, Calif., a corporation of Nevada Filed Aug. 23, 1965, Ser. No. 481,474 4 Claims. (Cl. 241-41) This invention relates to open circuit grinding of solids.

Grinding mills for reducing the size of solid particles may be open circuit mills or closed circuit mills. A closed circuit mill is one in which the material being ground passes through the mill and is subsequently classified so that oversize product is isolated and returned to the grinding mill feed. An open circuit mill is one in which the product passes through the mill just once, and the mill operation must be controlled to obtain the desired size distribution in the mill product in the single pass. The advantage of an open circuit mill is that it is not necessary to subject the full product stream to classification and recirculation of the oversize.

Open circuit mills can presently be controlled by varying the feed rate, the amount of grinding medium charge, or the speed at which the mill rotates. When too much oversize appears in the product reducing the feed rate, increasing the grinding medium charge, or reducing the speed of the mill within its operating range, will reduce the average size of the product.

This invention provides an open circuit mill with a control system for producing particles in a desired range of sizes. The device of this invention includes a grinding mill of any suitable type which is capable of reducing large particles to smaller particles, and a variable rate solid-s feeder which may feed solids per se or solids entrained in a fluid, for example as an aqueous slurry. The solids are fed to the mill at a rate regulated by the variable rate feeder and are ground while passing through the mill.

The mill product passes from the mill to recovery equipment, typically flotation machines. In the operation of the present invention a pilot stream is removed from the main product stream and analyzed for particle size distribution. The pilot stream is very small in volume compared with the mill product stream. Good sampling techniques should be used to select a pilot stream that is representative of the product being produced.

The pilot stream is passed to a size classification device and classified into streams of larger particles and smaller particles at any useful division point. Three streams are then available for control; the pilot stream, the stream of large particles and the stream of small particles. Any two of the streams may be used as control streams. The control streams are metered to determine their total volume and density, and these values are integrated to determine the total solid content. Control of the mill feed is based on the ratio of the integrated values of small to large particles or the ratio of either small or large particles to the pilot stream which includes all particles. The values for the control streams are fed into a controller which is capable of holding a set point relationship between the control streams by adjustment of the solids feed rate.

The output from the controller regulates the feed rate to the mill by suitable adjustment of the feeding means and it is varied so that the particle feed rate is inversely proportional to the quantity of particles in the stream of larger particles from the size classifier or directly proportional to the quantity of small particles from the classifier. With either manner of controlling, as the amount of large particles in the product increases the feed rate to the mill decreases and the average particle size of the product becomes smaller.

3,351,293 Patented Nov. 7, 1967 This invention can be best described with reference to the accompanying drawing which illustrates a schematic representation of one device embodying the inventlon.

Referring to the drawing, solid feed from bin 10 is withdrawn by feeder 11 and mixed with water added from line 12 and controlled by valve 13. The water is controlled as described hereinafter to provide a slurry of the proper density for grinding. The slurry is either formed before grinding, or it is formed directly in the mill 15. Feeder 11, the regulation of which will be described hereinafter, regulates the rate at which the solids are passed into mill 15.

Mill 15, which may be a ball mill or rod mill or other conventional grinding device, is operated at constant speed, and in passing through the mill from the inlet to the mill product stream, the solids are ground. The mill product passes through line 16 to equipment for beneficiating, filtering, drying or otherwise preparing it for its ultimate use.

Closely adjacent the outlet of the mill, sample cutter 17 Withdraws a small portion of the mill product called the pilot stream and pump 18 passes the pilot stream through line 20 containing flow meter 21 and densimeter 22, into a classification device 23. In classifier 23 the pilot stream is divided into a stream containing large particles discharged through line 25 and a stream containing small particles discharged through line 26. The stream of large particles in passing through line 25, is metered through a means 27 for measuring the solids content of the stream, and through a means 28, which measures the total flow of the stream after which the line 25 passes back into the main product stream in line 16. The stream of smaller particles in line 26 similarly passes through a means for measuring the solids content shown at 30, and a flow meter 31 after which it, too, passes back into line 16 rejoining the total mill product.

The broken lines'in the figure indicate input into controller 35, the input from each of the measuring devices being consistent with the device that is employed with it. The input may be varying electrical, pneumatic, hydraulic, or other energy forms which are proportional to the condition being sensed. It may also be emphasized here that all of the controls illustrated need not be used and in alternative modes of operating the invention it is sufficient to use only two of the illustrated streams for control. In one mode of operation which is perhaps the simplest, the steady operation of pump 18 provides an unvarying volumetric flow through the line 20. In this method there is no need to measure the volume flow as with fiow meter 21. However, the density in line 20 must be measured as with instrument 22 in order to compare the total solids content of the pilot stream with whichever stream from the cyclone is to be used as the second control stream. As illustrated, the controller 35 is capable of accepting input from densimeter 22 through line 36, in conjunction with measuring the fioW rate at flow meter 21 and fed to the controller through line 38. Lines 40 and 41 transmit an input from flow meter 31 and densimeter 30, respectively, which are employed to meter the stream containing large particles. Similarly, the measurements from flow meter 28 and densimeter 27 provide input to controller 35 through lines 42 and 43. respectively. Input from all of the sensing means is not necessary in this invention. For example, if controller 35 is to operate on the ratio of large particles to small particles, the input through lines 40, 41, 42 and 43 will operate the controller. Then the rate at which solids are fed through feeder 11, operated by a control impulse passed from controller 35 through line 45, is controlled by the relationship between the amount of larger particles in line 25 to the amount of smaller particles in line 26, comparing that relationship with a predetermined standard, i.e., the setpoint, and regulating the operation of feeder 11 to change the solids feed rate consistent with variations from the setpoint.

The controller 35 mayalso function by comparing the total solids input in line 20with the either amount of larger particles in line 25 or the amount of smaller particles in line 26. Since control of the mill of this invention is effected with two of the three streams available, means for producing discontinuity in the control lines are provided. These means are selected to be appropriate to the mode of transmitting signals, valves for pneumatic or hydraulic lines or switches for electrical, and are shown schematically as means 56, 58, 60, 61, 62 and 63 in lines 36, 38, 40, 41, 42 and 43 respectively. It is contemplated that at any time only four of the six lines will be operative.

Regardless of how the controller 35 functions it is evident that the device of the present invention provides an open circuit grinding. mill which does not require classification of the full millproduct streamand recirculation of oversize particles. The device of the present invention also does not require variable speed mill drive but effects control by the solid feed rate. The schematic drawing illustrates only one mode of controlling the solid feed rate although others may be used. The drawing also illustrates schematically thatthe liquid feed rate should be related ot the solid feed rate, and to this end a serial control system is shown wherein valve 13 is controlled through line 65 responsive to variations in the rate at which the solids feeder 11 is functioning. The liquid feed rate may also be controlled directly by controller 35, or independently by sensing the particle loading in the feed or product streams.

Conventional and commercially available sensing devices and controllers are contemplated for use inthis invention. Flow rate measuring devices such as flow meters, Venturi meters, Pitot tubes, positive displacement meters, etc. may be employed where indicated as 21, 28 and 31 in the schematic drawing, and densimeters 22, 28 and 30 may be any known means for measuring the solids content of a flowing stream.

Having thus described the invention,

What is claimed is:

1. An open circuit device for grinding solids which comprises:

(A) a mill for grinding solids,

(B) means for feeding solids to the mill at a variable rate,

(C) a classifier for separating a stream of solids entrained in a fluid into a stream of larger particles and stream of smaller particles,

(D) means for passing a pilot stream of ground solids entrained in a fluid stream from the mill to the classifier,

(E) means for measuring the solids content and flow rate of at least two of the pilot stream, the stream of larger particles and the stream of smaller particles,

(F) means for transmitting signals proportionate to said measurements to control means for:

(i) determining the total quantity of particles in each of the measured streams,

(ii) comparing the relationship of particle quantity in the control streams with a predetermined standard, and

(iii) regulating the solid feeding means to maintain the relationship at the predetermined standard.

2. A method for grinding solids which comprises:

(A) passing a mixture of fluid and solids into a constant-speed grinding mill,

(B) passing a mill product stream containing ground solids from the mill,

(C) passing a pilot stream from the mill product stream to a classifier which separates the pilot stream into a stream of larger particles and a stream of smaller particles,

(D) measuring the totalparticle content of two of the pilot stream, the larger particle stream, and the smaller particle stream and passing the resultant.

measurements as input to a controller which determines the relationship between the measurements,

(E) comparing the relationship between the measurements with a predetermined standard, and,

(F) regulating the solids feed rate to be directly responsive to the quantity of smaller particles when said relationship does not conform to the standard.

3. The method of claim 2 whereinthe fluid content of said mixture of fluid andsolids is regulated by the rate of solid feed.

4. The method of claim 2 wherein control of the solid feed rate is responsive to the relationship between the particle content of the larger particle stream and smaller particle stream.

References Cited UNITED STATES PATENTS 2,499,347 3/1950 Adams 241 34 2,930,347 4/1961 C1368. 3,094,239 6/1963 Fahlstrom et al. 241-34 3,145,935 3/1964 Wilson 241-34 3,248,061 4/1966 Franz 241-34 FOREIGN PATENTS 667,001 7/1963 Canada.-

WILLIAM W. DYER, JR., Primary Examiner.

G. A. DOST, Assistant Examiner. 

1. AN OPEN CIRCUIT DEVICE FOR GRINDING SOLIDS WHICH COMPRISES: (A) A MILL FOR GRINDING SOLIDS, (B) MEANS FOR FEEDING SOLIDS TO THE MILL AT A VARIABLE RATE, (C) A CLASSIFIER FOR SEPARATING A STREAM OF SOLIDS ENTRAINED IN A FLUID INTO A STREAM OF LARGER PARTICLES AND STREAM OF SMALLER PARTICLES, (D) MEANS FOR PASSING A PILOT STREAM OF GROUND SOLIDS ENTRAINED IN A FLUID STREAM FROM THE MILL TO THE CLASSIFIER, (E) MEANS FOR MEASURING THE SOLIDS CONTENT AND FLOW RATE OF AT LEAST TWO OF THE PILOT STREAM, THE STREAM OF LARGER PARTICLES AND THE STREAM OF SMALLER PARTICLES, 